We do have the reputation system enabled, you can give negative, positive, and neutral ratings although I'll be replacing that soon with hopefully a like system.
There is one on my forum, but I don't think it is used too often. There's positive, negative, and neutral. It's either +1, +2, -1, -2, or 1 and 2. You can add comments and all that aside the rating. I believe there is a way to report ratings too.
Generally, negative rep isn't worth the hassle, particularly in large forums where people are passionate about such things. I like the rep systems where a thumbs down counts as a "+0," such as the XenForo mod that lets you choose between a number of different responses, some of which are positive and some of which don't effect the rating.